An optical fiber including silica based glass containing an alkali metal element doped in a core area is known (see Patent Literatures 1 to 9). When the alkali metal element is doped in a core portion of an optical fiber preform, viscosity of the core portion can be reduced when the optical fiber preform is drawn into an optical fiber, and relaxation of a network structure of the silica glass progresses. Thus, transmission loss of the optical fiber may be reduced.
A diffusion method is known as a method of doping the alkali metal element into a silica glass (e.g., see Patent Literatures 1 and 2). In the diffusion method, the alkali metal element is diffusively doped to an inner surface of a glass pipe by heating the glass pipe using an external heat source or producing plasma inside the glass pipe while introducing a raw material vapor such as the alkali metal element or an alkali metal salt, which is a raw material, into the glass pipe.
After the alkali metal element is doped to the vicinity of the inner surface of the glass pipe in this way, this glass pipe is heated so that its diameter is reduced. After the reduction of the diameter, a certain thickness of the inner surface of the glass pipe is etched for the purpose of removing a transition metal element or the like (e.g., Ni or Fe) contaminated at the same time as the doping of the alkali metal element. After etching, a core rod containing an alkali metal element is manufactured by heating and collapsing the glass pipe. A cladding portion is synthesized onto an outer side of the core rod containing the alkali metal element to manufacture an optical fiber preform. Also, an optical fiber can be manufactured by drawing this optical fiber preform.